The invention relates to a mechanical/electrical transducer which has a bridge circuit formed by electrically connecting strain-sensitive thick-film resistors by means of conductor tracks, the thick-film resistors being arranged directly on a metallic component to be subjected mechanically to torsional loading and lying outside [lacuna] an axis of the component to be loaded which is neutral with respect to bending moments, it being possible to tap an electrical signal corresponding to the strain of the thick-film resistors when the component is subjected to torsion.
A mechanical/electrical transducer of the introductory-mentioned type is known from German Patent Application 198 14 261.7. In the case of this transducer, the strain-sensitive thick-film resistors are arranged directly on a shaft designed as a supporting element. Here, the shaft is subject to a mechanical load in the form of a torsion and the resulting surface strain is picked off by the resistor arranged without an intermediate support on this shaft. The thick-film resistors are here applied to the shaft by a printing method in the form of a resistor paste and are intimately connected to this shaft after a heat treatment.
Ideally, a torque sensor of this kind is intended to measure only torsional moments and to compensate for bending moments and tensile or compressive forces, i.e. no electrical signal should be output at the bridge circuit in the case of bending moments, tensile and compressive forces. If the thick-film resistors of the bridge circuit are arranged outside a zone of the shaft which is free from forces as regards bending moments (neutral axis for bending moments), there is the risk that an unwanted bridge signal will be produced in the case of bending moments and will falsify the sensor signal.
The object on which the invention is based is therefore to specify a mechanical/electrical transducer in which the electrical signal is tapped with the aid of a bridge circuit but the production of a bridge signal due to bending moments or tensile and compressive forces is reliably prevented.
According to the invention, the object is achieved by virtue of the fact that in each case at least one thick-film resistor in a first arm of the bridge circuit is at the same distance from and at an angle of the same magnitude to the neutral axis of the component to be loaded as the opposite thick-film resistor in the second arm of the bridge circuit.
The invention has the advantage that, when bending moments occur, the thick-film resistors change their resistance value uniformly, the unwanted signal thereby being compensated when they are connected up to form a measuring bridge. This is achieved with the aid of bending-compensated positioning of the thick-film resistors.
As an alternative, the object is achieved by virtue of the fact that the thick-film resistors arranged in one bridge arm are at the same distance from and at an angle of the same magnitude to the neutral axis of the component to be loaded.
In a refinement, the component to be loaded has on its surface an aperture which, when the component is mechanically stressed in at least one area of the surface of the component in which the thick-film resistors are positioned, a relationship of unequal magnitude between the longitudinal and the transverse strain is produced.
The aperture simply boosts the signal behavior of the sensor without any complex changes to the shaft geometry. A sensor of this kind is suitable for mass production since it can be produced economically and in a short time. By virtue of the aperture, the mechanical stresses acting on the component are superimposed, the strain in the principal directions (longitudinal, transverse) being of unequal magnitude, allowing amplification of the signal behavior of the sensor.
It is advantageous if the aperture is designed as a through opening in the component.
In a development, the opening is designed as a slotted hole, the first thick-film resistor of a bridge arm being arranged in the vicinity of a first radial zone of the slotted hole, and the first thick-film resistor of the second bridge arm being arranged in the vicinity of a second radial zone of the slotted hole, the signal being tapped from the bridge arm between the thick-film resistors arranged in the different radial zones.
This circuit configuration on the one hand ensures that the two thick-film resistors situated opposite one another in the bridge arm are at the same distance from the neutral axis and the half-bridge voltage of the two bridge arms change uniformly under the influence of bending moments.
A reliable, identical change in the bending moments of the thick-film resistors is achieved if the thick-film resistors of both bridge arms are arranged in the radial zones above or below the longitudinal extent of the component.
The mechanical/electrical transducer is particularly simple to produce if the thick-film resistors are arranged on a flat surface of the component, which is produced from steel or steel alloys. Since the thick-film resistors are arranged directly on the component to be mechanically loaded and an intermediate support is omitted, the mechanical load to be detected is picked off directly from said component without signal distortions due to the intermediate support. Producing the thick-film resistors directly on the component to be loaded considerably reduces production costs.
To minimize the offset (zero offset) due to production within the bridge itself, a thick-film balancing resistor, the resistance value of which is lower than that of the thick-film resistors, is arranged in each bridge arm. The bridge-circuit output signal obtained in this way makes possible a high gain of the amplifier connected downstream of the mechanical/electrical transducer. At the same time, it allows high resolution of the signal by an A/D transducer where the measurement signal is digitized.
The thick-film balancing resistor of one bridge arm is at the same distance from and at an angle of the same magnitude to the neutral axis of the component as the thick-film balancing resistor of the other bridge circuit. Thus, compensation for the bending moments is achieved by appropriate positioning in the case of the balancing resistors too.
It is advantageous if, for gradation of the resistance, the thick-film balancing resistor comprises. resistance tracks connected in parallel which are severed sequentially during the balancing process.
Digital balancing in this way continues until the offset is at the desired minimum value. This balancing method ensures the long-term stability of the output electrical signal.
In order to make the screen contact surface and distribution of forces during screen printing as uniform as possible, the bridge circuit has a symmetrical layout for production by screen printing. In this case, the resistance value obtained for all the thick-film resistors to be printed is as uniform as possible in order to match the film thickness of the thick-film resistors.
In order to improve the printing accuracy of the resistance values, a strictly symmetrical layout is achieved by arranging stub lines and/or reactors on the surface of the component.
To enable the mechanical/electrical transducer to be used, in particular, for applications critical for safety, such as steering assistance in motor vehicles, a first bridge circuit is arranged above the aperture and a second bridge circuit is arranged below the aperture.